1. Field of the Invention
The present invention relates to a die-expanded molded foaming apparatus for forming molded foam products using material beads made of thermoplastic synthetic resin, such as polyolefine and polystyrene, and a method therefor.
2. Description of the Related Art
As a die-expanded molded foaming apparatus for manufacturing molded foam products using material beads made of thermoplastic synthetic resin, an apparatus shown in FIG. 10 has been commercialized, wherein a set of dies 100 and 101 are disposed opposite each other, chambers 102 and 103 are disposed at the rear side of the dies 100 and 101 respectively, many vent holes 105 and 106 connecting the chambers 102 and 103 and the cavity 104 are formed in both of the dies 100 and 101, and utility fluid, such as the later mentioned heating steam, is supplied to the cavity 104 or exhausted from the cavity 104 through the vent holes. In the structure of this case, upper utility ports 107 and 108 for supplying heating steam are disposed at the upper part of the respective chambers 102 and 103, and the lower utility ports 109 and 110 connected to a pressure reducing pump and a drain pipe are disposed at the lower part of the respective chambers 102 and 103 so that steam is supplied to the cavity 104.
For actually forming the many vent holes 105 and 106 opened in the dies 100 and 101, a core vent 111, which is a cylindrical body with a cap having an outer diameter of 7-12 mm where a plurality of vent holes 105 and 106, which are approx. 0.5 mmxc3x8 round holes or approx. 0.5 mm width slits, are opened, is embedded in the respective core vent attachment holes 112, which are opened at a 20-50 mm pitch in the dies 100 and 101, as illustrated in FIG. 11 and FIG. 12.
Using such a die-expanded molded foaming apparatus, pre-expanded material beads are filled into the cavity 104, are heated with the heating steam for foaming and fusing, are cooled and solidified, and then taken out as a molded foam body in a predetermined shape, and the role of the vent holes 105 and 106 in this molded foaming will now be further explained.
In the Japanese Patent Laid-Open No. 57-174223, the process drawing shown in FIG. 13 is included, wherein FIGS. 13(a)-13(d) depict the pre-heat exhausting process for substituting air in the dies and air among the material beads with steam, and the specific content of each process will be explained below. In FIG. 13, a black valve symbol indicates that the valve is closed, and a white valve symbol indicates that the valve is open.
FIG. 13(a) shows an exhaust process, wherein after the material beads are filled in the cavity 104, steam is supplied from the top utility ports 107 and 108 to the chambers 102 and 103 for a very short time, and at the same time, air in the dies, particularly in the chambers 102 and 103, is exhausted from the bottom utility ports 109 and 110 by suction. In this case, pressure inside the chambers 102 and 103 is increased to be a plus pressure by steam so that steam enters among the material beads through the vent holes 105 and 106.
FIG. 13(b) shows a both-side exhaust process, wherein the top utility ports 107 and 108 are closed, and the vacuuming and pressure reducing operation is continued to reduce pressure inside the dies so that air existing in spaces among the material beads is sucked and exhausted through the vent holes 105 and 106 formed in the dies on both sides.
FIG. 13(c) shows a one-side preheating process, wherein the bottom utility ports 109 and 110 are closed and steam is supplied from the top utility port 108 of one chamber 103, which is in a pressure- reduced state, for a short time. In this case, the supplied steam passes sequentially through the vent holes 106 of the die 101, among the material beads in the cavity 104, and the vent holes 105 of the die 100, and flows into the chamber 102 at the opposite side, by which all of the material beads and the dies 100 and 101 are preheated.
FIG. 13(d) shows a one-side preheating process with an opposite flow of steam, wherein, a similar operation is carried out from the chamber 102 side so that air in the cavity 104 is completely exhausted, and at the same time, both dies 100 and 101 are preheated while decreasing the localized temperature difference as much as possible.
FIG. 13(e) shows a heating process for fusion, wherein steam for heating for fusion is supplied to both of the chambers 102 and 103 for heating the dies 100 and 101, and for heating the materials beads as well through the vent holes 105 and 106 of the respective dies 100 and 101, so as to complete foaming, and to mutually fuse the material beads to form the molded foam body.
The vent holes formed in the dies in this manner play an important role as exhaust passages of the air among the material beads or as supply passages of heating steam for obtaining a homogeneous molded foam body, but on the other hand, the following problems have also been recognized.
(1) In order to compensate for strength which is decreased by opening many core vent attachment holes in the dies, the wall thickness of a die made of aluminum alloy material must be set thick, 8-12 mm for example, which increases heat capacity, causing such problems as poor heat efficiency for heating and cooling, or a slow speed of temperature rising and lowering, which drops control accuracy.
(2) Since 2000-4000 core vent attachment holes are opened in a general pair of dies, a complicated drilling operation increases processing cost, and since the core vents are attached manually, this operation is complicated, causing inevitable damage on the surface of the dies, where an extra operation for repair is necessary.
(3) Vent holes are clogged by e.g. scales, causing heat failure, mold releasing failure, and cooling failure, which makes a maintenance operation necessary, such as replacing core vents or periodic cleaning by high pressure cleaning water.
(4) Traces of core vents and vent holes remain on the surface of the molded foam product, which causes a drop in the beauty of the appearance of the molded foam product, and when the outer surface is printed, traces of core vents and vent holes become the cause of dropping printing quality.
(5) After molding, the molded foam product is cooled down by spraying cooling water into the chambers, and at this time moisture infiltrates into the cavity through the vent holes, resulting in the molded foam product containing 6-10% water inside, which makes a drying process necessary. Also the cooling water must be controlled to be a clean state to obtain a clean molded foam product, since the cooling water directly contacts the molded foam product.
(6) Since the material beads are heated for expanding and fusing under the same heating conditions by supplying steam from the chambers to the cavity, the surface property of the molded foam product obtained like this (hereafter equal heating molded foam product) changes depending on the fusion rate of the beads. Specifically, the surface property worsens as the fusion rate decreases, and surface property improves as the fusion rate increases. On the other hand, the at higher the fusion rate of the beads is set for an equal heating molded foam product, the better properties become, such as the mechanical strength of the molded foam product, but heating, expanding and fusing time, and cooling time become longer, which makes the general cycle time of molding longer, decreasing productivity.
For the above reasons, in the above mentioned molding technology, the fusion rate of beads of a molded foam product is set to e.g. 40-80% so as to improve surface property to insure beauty in appearance and to insure mechanical strength by setting a sufficiently high fusion rate, however the fusion rate must be set high enough to insure beauty in appearance even for a molded foam product which does not demand high mechanical strength, therefore cycle time for molding increases and productivity decreases. The fusion rate here is based on an evaluation of the state of beads in a cross-section when the molded foam product is split, and more specifically, the ratio of broken beads determined by measurement, regarding a bead as not fused when the head itself is not broken but has a crack along its surface, and regarding a bead as fused when the bead itself is broken.
It is an object of the present invention to provide a method for die-expanded molded foaming of synthetic resin and die-expanded molded foam products so as to adjust the internal fusion rate of the molded foam product while improving the beauty in appearance of the molded foam product, and to implement both productivity and commercial value.
A first aspect of the present invention is a die-expanded molded foaming apparatus of synthetic resin, wherein the first openings opening to a cavity are formed in a molding section of a set of dies for molding a molded foam product, which is to mold a non-outstanding portion of the molded foam product, and connecting passages for connecting the first openings and external utility pipes are disposed.
In accordance with this molded foaming apparatus, material beads made of thermoplastic synthetic resin material are filled in the cavity enclosed by the set of the dies, and in this state, the material beads are heated, expanded and fused by supplying heating steam into the cavity, but traces of the first openings do not reduce the beauty in appearance of the molded foam product since the first openings are formed only in a molding section of the dies which is used to mold a non-outstanding portion of the molded foam product.
For the raw material of the material beads, a raw material having characteristics suitable for the working conditions of the molded foam product to be manufactured is selected, where polystyrene synthetic resin material, polyolefine synthetic resin materials such as polyethylene and polypropylene, and a copolymer of these synthetic resin materials can be used. Material beads made of polyolefine synthetic resin material, in particular, are preferable in terms of improving filling property, because the raw material itself is soft and has high gas permeability, which makes it much easier to change the shape of particles than material beads made of polystyrene synthetic resin material having the same expansion ratio.
A second aspect of the invention is the molded foam apparatus, wherein chambers independent from the cavity are formed on the rear side of both of the above mentioned dies, and these chambers and the cavity are connected to utility pipes as separate systems.
In accordance with this molded foaming apparatus, the two chambers at the rear side of the dies are constructed in an independent space from the cavity, and respective heating conditions can be adjusted independently, which allows adjusting the temperatures of the set of the dies independently by steam to be supplied to the two chambers so as to adjust the surface property of the molded foam product contacting the dies, and to adjust the fusion rate of the material beads independently from the surface property by heating, expanding and fusing the material beads filled in the cavity by steam to be supplied to the cavity. This makes it possible to manufacture a molded foam product with a beautiful surface while keeping the internal fusion rate of the molded foam product low to decrease the cycle time of molding, and to implement both productivity and commercial value. Also the cavity and the chambers are not connected by the first opening, which prevents cooling water sprayed in the chambers at cooling from contacting the molded foam product, and to prevent the rise of the moisture content of the molded foam product. The cooling water does not directly contact the molded foam product, which makes it possible to obtain a clean molded foam product without strictly controlling the cooling water to be a clean state.
A third aspect of the present invention is the molded foaming apparatus, wherein the above mentioned first openings are formed in a slit shape along the joint of the set of the dies. According to this structure, the number of core vent attachment holes to be formed in the dies can be minimized or eliminated, which makes it possible to constitute the dies to be thin, decreasing manufacturing cost, and improving the control accuracy of utility fluid, such as steam. Also operations for forming core vent attachment holes and core vent attachment operations are decreased or eliminated, which makes it possible to improve the productivity of the dies, and to decrease or eliminate the maintenance operation required due to the clogging of the vent holes by scales. Also the traces of the core vents and vent holes to be formed on the molded foam product are decreased or eliminated, which further improves the beauty in appearance of the molded foam products.
It is preferable that the connecting passages for connecting the first openings in a slit shape and the external utility pipes are formed from the cavity side to the outside, along the joint of the dies, which is a fourth aspect of the present invention, or the first openings are formed near the joint of the set of the dies, which is a fifth aspect of the present invention. In other words, the joint of the dies is normally formed at the outer edge of the molded foam product, so forming the first openings in such a location is preferable since steam can be supplied evenly in the cavity.
A sixth aspect of the present invention is the molded foaming apparatus wherein the above utility passages are formed at a location which does not include the joint of the dies, and the first openings formed near the joint are opened to these utility passages. If the first openings are formed so as to include the joint of the dies, then the joint portion is worn out by the steam which enters into the cavity from the joint portion, so, to prevent this, it is preferable to form the first openings at a location which does not include the joint.
A seventh aspect of the present invention is the molded foaming apparatus, wherein the first openings are formed in a portion which is exposed to the cavity of an accessory part attached to at least one of the dies. The possible accessory part is at least one of a material beads filling unit and an ejector pin, which is an eighth aspect of the present invention. In other words, traces of the end faces of the filling unit and the ejector pin are always formed on the surface of the molded foam product, and forming the first openings near the filling unit or the ejector pin makes the trace of the first openings non-outstanding, and prevents deterioration of the appearance of the molded foam product.
A ninth aspect of the present invention is the molded foaming apparatus, wherein a plurality of the first openings are formed and divided into two groups, so that the utility fluid can be supplied from the first openings of one group to the first openings of the other group via the cavity. The steam may be supplied and exhausted by connecting independent utility pipes to each one of the first openings, but since a large number of the first openings must be formed to supply steam evenly into the cavity, it is preferable to divide the first openings into two groups, so as to supply the utility fluid from the first openings of one group to the first openings of the other group via the cavity. This is preferable, particularly because operation to substitute air among the material beads filled in the cavity with steam or operation of heating the material beads can be carried out quickly.
A tenth aspect of the present invention is the molded foaming apparatus, wherein second openings opening to a portion where the flow of the utility fluid becomes non-uniform in the cavity, are formed in the dies, and the connecting passage for connecting the second openings and the external utility pipes are disposed. In a portion where the flow of the utility fluid is non-uniform, a density unevenness in filling the material beads and heating and cooling non-uniformity is apt to occur, so it is preferable to form the second openings for dedicated supply and exhaust of the utility fluid in such a portion. A possible example of a portion where the flow of the utility fluid becomes non-uniform is the inner part of a concave area when the concave area is formed in the dies, and in this case, it is preferable to form the second openings in the inner part of such a concave area, which is an eleventh aspect of the present invention.
A twelfth aspect of the present invention is the molded foaming apparatus, wherein a pair of the first openings in a slit shape are formed, and when the space between the first openings is 100 mm or more, the second openings are formed on a surface of the die at the intermediate position of the first openings. When the space between the first openings is wide, the flow resistance of the utility fluid increases which makes the flow of the utility fluid non-uniform, so it is preferable to form the second openings in such a location.
The thirteenth aspect of the present invention is the molded foaming apparatus, wherein the second openings and the first openings are connected to the utility pipes as different systems. It is also possible to connect the first openings and the second openings to the utility pipe as a same system, but separate systems make it possible, for example, to first fill the material beads into the concave portion, where filling the material beads is difficult, so as to control the filling density of the material beads to be even, which makes it possible to manufacture molded foam products with better quality.
A fourteenth aspect of the present invention is the molded foaming apparatus, wherein the width of the opening of the first openings and the second openings are set to the outer diameter or less of a material bead to be filled. If the width of the opening of the first and second openings exceed the outer diameter of a material bead, the material beads flow out of the cavity through the openings, so the width of the opening of both openings is set to the outer diameter or less of a material bead.
A fifteenth aspect of the present invention is the molded foaming apparatus, wherein the first openings and the second openings are formed by disposing a replaceable opening forming element to the dies. Since such a utility fluid as heating steam flows through the openings, the abrasion of the openings is inevitable. Therefore a replaceable opening forming element may be disposed at the dies so that the opening forming element can be replaced when the openings are worn out.
A sixteenth aspect of the present invention is a method for die-expanded molded foaming of synthetic resin, wherein using the die-expanded molded foaming apparatus of synthetic resin according to one of the first to fifteenth aspects where the chambers independent from the cavity are formed on the rear side of the set of dies respectively so as to supply steam as the utility fluid independently to the cavity and both chambers, the heating conditions for both of the chambers and the heating conditions for the cavity are controlled in a state where material beads made of thermoplastic synthetic resin are filled in the cavity, and the material beads filled in the cavity are heated, expanded and fused while controlling the internal fusion rate at will with maintaining the surface property of the molded foam product.
In accordance with this molded foaming method, wherein the heating conditions in the cavity and a set of the chambers are independently controlled, the surface property of the molded foam product contacting the dies can be adjusted by adjusting the temperature of the set of the dies independently by the steam to be supplied to the two chambers, and the fusion rate of the material beads can be adjusted independently from the surface property by heating, expanding and fusing the material beads filled in the cavity by the steam to be supplied to the cavity. This makes it possible to decrease the cycle time of molding while keeping the internal fusion rate of the molded foam product low, and to manufacture molded foam products with a beautiful surface as well, consequently both productivity and commercial value can be implemented.
A seventeenth aspect of the present invention is a method for molded foaming, wherein a plurality of the first openings are divided into two groups, and the internal fusion rate is controlled at will with maintaining the surface property of the molded foam product, while independently controlling the four heating conditions, that is, the heating conditions for the cavity and the heating conditions for both of the chambers for these two groups. Heating conditions for the set of the chambers may be controlled together, but it is preferable to control the respective chambers independently since the surface property of a surface at one die side and the surface property of a surface at the other die side of the molded foam product can be controlled independently.
An eighteenth aspect of the present invention is a method for molded foaming, wherein the steam pressure and time are controlled as heating conditions. It is also possible to control the steam temperature as a heating condition but controlling the steam pressure and time is preferable, since conventional steam equipment which has been used at a factory can be used, and equipment for control can be constructed at low cost.